Electronic Physician (ISSN: 2008-5842) http://www.ephysician.irJanuary 2016, Volume: 8, Issue: 1, Pages: 1752-1758, DOI: http://dx.doi.org/10.19082/1752

Corresponding author:Mohammad Zakaria Nassani, Department of Prosthetic Dental Sciences, Al Farabi College for Dentistry andNursing - Riyadh –Saudi Arabia. Tel: +96.6112269387, Fax: +96.6112324580, Email: mznassani@hotmail.comReceived: November 09, 2015, Accepted: December 29, 2015, Published: January 2016iThenticate screening: November 22, 2015, English editing: January 03, 2016, Quality control: January 06, 2016© 2015 The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use isnon-commercial and no modifications or adaptations are made.

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The effect of flexible acrylic resin on masticatory muscle activity in implant-supported mandibularoverdentures: a controlled clinical trial

Eman Mostafa Ahmed Ibraheem1, Mohammad Zakaria Nassani2

1 Fixed and Removable Prosthodontics Department, Oral and Dental Research Division, National Research Centre,Giza, Egypt2 Department of Prosthetic Dental Sciences, Al Farabi College for Dentistry and Nursing, Riyadh, Saudi Arabia

Type of article: Original

AbstractBackground: It is not yet clear from the current literature to what extent masticatory muscle activity is affectedby the use of flexible acrylic resin in the construction of implant-supported mandibular overdentures.Objective: To compare masticatory muscle activity between patients who were provided with implant-supportedmandibular overdentures constructed from flexible acrylic resin and those who were provided with implant-supported mandibular overdentures constructed from heat-cured conventional acrylic resin.Methods: In this clinical trial, 12 completely edentulous patients were selected and randomly allocated into twoequal treatment groups. Each patient in Group 1 received two implants to support a mandibular overdenture madeof conventional acrylic resin. In Group 2, the patients received two implants to support mandibular overdenturesconstructed from “Versacryl” flexible acrylic resin. The maxillary edentulous arch for patients in both groups wasrestored by conventional complete dentures. For all patients, masseter and temporalis muscle activity wasevaluated using surface electromyography (sEMG).Results: The results showed a significant decrease in masticatory muscle activity among patients with implant-supported mandibular overdentures constructed from flexible acrylic resin.Conclusion: The use of “Versacryl” flexible acrylic resin in the construction of implant-supported mandibularoverdentures resulted in decreased masticatory muscle activity.Keywords: flexible acrylic resin, muscle activity, implant-supported overdentures, electromyography

1. IntroductionThe superiority of implant-supported overdentures (ISODs) over the conventional prosthodontic treatment bycomplete dentures is well documented in the literature (1, 2). Enhanced denture retention, better denture stability,more efficient chewing function, and improved patient satisfaction are some of the benefits that can be obtainedfrom ISODs compared to complete dentures (3). While heat-cured conventional acrylic resin is the most populardenture base material, flexible acrylic resin can be used to achieve even force distribution, reduced localizedpressure, and improved denture retention by close adaptation to the supporting tissues and engagement of soft-tissueundercuts. Moreover, the flexible acrylic resin possibly can absorb the impact forces during functional and para-functional movements (4-5). Electromyography (EMG) is defined as the graphic recording of the electrical potentialof muscles. It has been the only tool used to assess the muscle activity of the stomatognathic system since it first wasintroduced by Robert Moyers in 1949. Over the years, clinicians and researchers have used electromyographicactivity to evaluate the chewing function in denture wearers. The EMG can be considered as a reliable tool for thediagnosis of neuromuscular pathology of the stomatogmathic system and temporomandibular joint disorder (TMJ)(6, 7). The current literature shows that the EMG technique provides valid and quantitative data for the assessmentof the functional condition of the masticatory muscles in rest condition, chewing, maximum muscle activation,bilateral symmetry of the contraction behavior of the jaw muscles, and also for the measurement of stomatognathic

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system reflexes (8). To date, it is not yet clear to what extent masticatory muscle activity can be affected by the useof flexible acrylic resin in the construction of mandibular ISODs. The aim of the present study was to comparemasticatory muscle activity between patients who were provided with mandibular ISODs constructed from flexibleacrylic resin and those who were provided with mandibular ISODs constructed from a rigid, heat-cured,conventional acrylic resin. The hypothesis that was examined was as follows: There is no significant difference inmasticatory muscle activity between mandibular ISODs of conventional construction and mandibular ISODs madeof flexible acrylic resin.

2. Material and Methods2.1. Study settingThis was a controlled clinical trial. Twelve completely edentulous male patients whose ages ranged from 45 to 60were selected from the out-patient clinic at the Fixed and Removable Prosthodontic Department, National ResearchCentre, Cairo, Egypt.

2.2. Inclusion criteriaAll patients were evaluated before inclusion in this study through history, clinical, and radiographic examination. Tobe included in the study, the patient had to meet the following inclusion criteria:

1) Good physical and mental health.2) The patient has been edentulous at least for five years with adequate inter-arch space to accommodate the

attachment system.3) Firm healthy alveolar mucosa free from inflammation or ulceration.4) Sufficient mandibular alveolar bone of good quality and quantity to support dental implants. (Digital

panoramic radiographs were used to assess the quality of the bone and to determine the possibility forinstallation of two dental implants in the symphyseal area.)

2.3. Exclusion criteriaThe following were set as the exclusion criteria of the study:

1) Patients were excluded if they had diseases that might negatively influence implant installation and/orOoseointegration (such as osteoporosis and diabetes).

2) The patient has a contraindication for the implant surgery (bleeding disorder, for example).3) Patients with para-functional habits (e.g., clenching and bruxism).4) Patients with TMJ disorders

2.4. Research ethicsThe study protocol was approved by the Ethics Committee of the National Research Center, Cairo, Egypt. Allpatients were informed thoroughly about the study, and each patient was asked to sign a consent form. Onlymotivated and cooperative patients who accepted the periodic recall visits and agreed to sign the consent form wereenrolled.

2.5. Patient groupingPatients were randomly and equally divided into two treatment groups:

1) Group 1: Each patient in this group received conventional maxillary and mandibular complete denturesmade of heat-cured acrylic resin. In the mandible, two implants were inserted in the symphyseal area andretained by ball-and-socket attachments.

2) Group 2: Patients in this group received the same type of treatment as the patients in group 1, but themandibular overdentures were made of the flexible acrylic resin “Versacryl” (Keystone Industries GmbH,Sigen, Germany).

2.6. Prosthetic proceduresFor all patients, complete upper and lower dentures were constructed in a conventional manner. Primary impressionswere made using alginate impression material (Chromaclone, Ultradent Products, Inc., South Jordan, UT). Finalimpressions were made using rubber base impression material (Zhermack, Via Bovazecchino, Badia Polesine (RO)Italy). Jaw relation records were made using the wax wafer technique (Cavex Holland Bv, Haarlem, Noord-Holland,Netherlands). The maxillary cast was mounted on a semi-adjustable articulator (Hannau, Modd H, Teledyne,Buffalo, NY, USA) by using the maxillary face-bow record, while the mandibular cast was mounted by the centricjaw relation record. Modified anatomic acrylic resin artificial teeth (Acrostone Dental & Medical Supplies, Cairo,

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Egypt) were used and set up according to the lingualized occlusion concept (9). Plaster indexes were made for themaxillary trial dentures for clinical remounting procedures. The waxed-up dentures were placed in flasks andprocessed into heat-cured acrylic resin (Acrostone Dental & Medical Supplies, Cairo, Egypt) using a long curingcycle (9 hr at 70 °C). However, for patients in group 2, the mandibular overdentures were made of flexible acrylicresin “Versacryl” (Keystone Industries GmbH, Sigen, Germany).

2.7. Surgical proceduresPreoperative digital panoramic radiographs were made to assess the proposed implant sites with the aid of aradiographic stent. The final processed dentures were duplicated into clear acrylic resin templates to be used assurgical stents. All patients received antibiotic prophylaxis 2 g/d of Amoxicillin Clavulanic acid 24 hr prior tosurgery and until the fifth day postoperatively. Two root form endosseous implants (Zimmer Dental, Tapered SwissPlus, Implant system, Warsaw, IN, USA) were installed in the mandibular canine areas for each patient afterpreparation of the implant bed using pilot, intermediate, and final drills.

2.8. Loading of the implantsThe implants were allowed to osseointegrate for four months; then, ball and socket attachments (Zimmer, Inc.ImplantPart, Warsaw, IN, USA) were utilized to retain the lower dentures. The metal housings of the ball and socketattachments were picked up in the fitting surface of the lower dentures by self-cured acrylic resin. The direct pick-uptechnique allowed proper placement of the cap relative to the supporting ball. Also, it avoided the distortion of thecaps during the heat curing of acrylic resin if they had been fixed to the denture before processing (10). Figure 1shows the picked-up female housings in a Versacryl mandibular denture.

Figure 1. A picked-up female housings into a Versacryl mandibular denture

2.9. Follow-up of the patientsPatients were instructed concerning proper oral health and denture hygiene and asked to return for subsequent recallvisits. At each review appointment, patients’ complaints were recorded. The dental implants, the supporting tissues,the denture surfaces, and the occlusion and articulation of the dentures were examined. Then, the dentures wereadjusted based on the findings of the clinical examination and the patients’ complaints.

2.10. Evaluation of masticatory muscle activityFor all of the patients, masseter and temporalis muscle activity was evaluated using surface electromyography(sEMG) (Nihon Kohden, America, Inc., Foothill Ranch, CA, USA). The mean electromyographic amplitudes ofboth the masseter and temporalis muscles were recorded using surface electrodes during maximum clenching andchewing soft and hard food. During the EMG recording appointments, the patients were seated in an upright, relaxedposition with their heads in the same line as their bodies. Before attaching the surface electrodes, the patient’s skinwas cleaned with alcohol at the corresponding areas of the masseter and temporalis muscles. Furthermore, wecleaned the lobule of the ear where the earth electrode had been placed. The inner sides of the electrodes were filledwith Ten20 conductive EEG paste 32 (Weaver and Co., Aurora, CO, U.S.A.) and were fixed on the participant’sskin using adhesive tape. Muscle activity was recorded during maximum clenching and when chewing equally-sizedpieces of carrot as hard food and banana as soft food. Measurements were displayed and saved on a computer. The

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records were obtained at denture insertion and at two, four, and six weeks after insertion of the denture. Figures 2and 3 illustrate one of the patients during the evaluation of masticatory muscle activity using surface EMG.

Figure 2. Electromyography evaluation of the masseter muscle. The surface electrode is fixed to the massetermuscle and the earth electrode is placed on the lobule of the ear

Figure 3. Elecrtomyographic evaluation of the anterior temporalis muscles. The surface electrode is fixed to theanterior temporalis muscles and the earth electrode is placed on the lobule of the ear

2.11. Statistical analysisStatistical analysis was performed with IBM SPSS statistical software, Version 20 for Windows. The independentsamples t-test was used to study the difference between Versacryl ISODs and conventional ISODs for all testedvariables. One-way ANOVA was used to study the effect of time on mean Electro-Myo-Gram (EMG) (mV) for the

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patients in each group. Tukey’s post-hoc test was used for pair-wise comparison between the means when the resultsof the ANOVA test were significant.

3. ResultsOver the follow-up period, no patients withdrew from the study, no implant failures were observed, and only minoradjustments were needed for the constructed overdentures.

3.1. Effect of flexible acrylic resin on masseter muscle activity1) Maximum clenching record: The results revealed a significant decrease in the mean EMG muscle activity

for Versacryl dentures at denture insertion and four weeks record at p-value = 0.010 and < 0.010,respectively. At the two-week and six-week records, there was a decrease in the mean EMG muscleactivity. However, this decrease did not reach a significant level.

2) Soft food record: The results indicated a significant decrease in the mean EMG muscle activity forVersacryl dentures at two-week and six-week records at p-values = 0.012 and 0.029, respectively. Also, aninsignificant decrease of the mean EMG muscle activity between the two study groups was found atdenture insertion and four-week records.

3) Hard food record: A significant decrease was found in the mean EMG muscle activity for Versacryldentures at two-, four-, and six-week records at p-values = 0.014, 0.025, and < 0.001, respectively.However, an insignificant decrease in the mean EMG muscle activity between the two study groups wasfound at denture-insertion record.

3.2. Effect of flexible acrylic resin on temporalis muscle activity1) Maximum clenching record: The statistical analysis demonstrated a significant decrease in the mean EMG

muscle activity for Versacryl dentures at denture-insertion, two-, four-, and six-week records at p-value ≤0.001.

2) Soft food record: The results revealed a significant decrease in the mean EMG muscle activity forVersacryl at denture-insertion, two-, four-, and six-week records at p-value≤ 0.001.

3) Hard food record: A significant decrease was found in the mean EMG muscle activity for Versacryldentures at denture-insertion, two-, four-, and six-week records.

3.3. Effect of time on masseter muscle activity1) Group 1 (Conventional ISODs): The statistical analysis did not identify any significant decrease in the

EMG muscle activity for the clenching records between two weeks, four weeks, and six weeks followingdenture insertion. However, the data revealed a significant decrease in EMG muscle activity betweendenture-insertion and six-week records at p-value = 0.026 for the clenching records. For the soft foodrecords, there was no significant difference in EMG muscle activity between the follow-up records (i.e.,two-week, four-week, and six-week records). Similarly, with hard-food records, no significant decrease inEMG muscle activity was detected between the follow-up records.

2) Group 2 (Versacryl ISODs): There was an insignificant decrease in the EMG muscle activity for theclenching records over the follow-up period. With soft food records, a significant decrease in the EMGmuscle activity was detected between the time of denture insertion and six-week records at p-value = 0.004.However, there was insignificant difference in the EMG muscle activity between two- and four-weekrecords for the soft food. With hard-food records, no significant decrease in the EMG muscle activity wasnoted between the two-, four- and six-week records. However, the data indicated that there was asignificant decrease in the EMG muscle activity between the time of denture insertion and the six-weekrecords at p-value≤ 0.001 for hard food.

3.4. Effect of time on temporalis muscle activity1) Group 1 (Conventional ISODs): There was no significant decrease in the mean muscle activity between the

two-, four- and six-week records for the clenching procedure. For the soft-food records, a significantdecrease in the mean muscle activity was noted between denture-insertion record and six-week record at p-value = 0.001. Similarly, with hard-food records, there was a significant decrease in the mean muscleactivity between the denture-insertion record and the six-week record at p-value = 0.003.

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2) Group 2 (Versacryl ISODs): For clenching records, a significant decrease in the mean EMG muscle activitywas recorded between the denture-insertion record and the six-week record at p-value ≤ 0.001. Also, thedata showed that there was a significant difference between two-, four- and six-week clenching records.Furthermore, for the soft-food records, there was a significant decrease in the EMG muscle activitybetween the denture-insertion record and the six-week record at p-value = 0.001. The results were similarwith hard-food records, i.e., a significant decrease in the mean muscle activity was noted between thedenture-insertion record and the six-week records at p-value = 0.003.

4. DiscussionAlthough the heat-cured acrylic resin is the most popular denture base material, this study reported the use offlexible acrylic resin “Versacryl” to construct mandibular overdentures supported by two dental implants. Theflexible acrylic resin first was introduced in 1950 as an alternative to conventional acrylic resin denture basematerial (11). It is a flexible, biocompatible, thermoplastic, denture-base material with unique physical and estheticproperties (12). The flexible acrylic can be adjusted simply by using warm water to soften the material so as toconform to the contours of the soft and hard tissues. Also, the flexible acrylic can be extended into undercut areas tomechanically retain the dentures. Furthermore, the softness of flexible acrylic provides a feeling of comfort to thepatient (11). However, the flexible acrylic has some disadvantages and may lose its flexibility over time (4, 13).Many brands of thermoplastic or flexible denture materials are available in the market, such as Valplast (ValplastInt. Corp., Westbury, New York, USA), Flexiplast (Bredent, Senden, Germay), Flexite (The Flexible Company,Mineola, New York, USA), and Lucitone® FRS™ (DENTSPLY International, Woodbridge, Ontario, Canada). Inthis study, Versacryl (Keystone Industries GmbH, Sigen, Germany) was used as a thermoplastic material toconstruct mandibular overdentures supported by two dental implants. Versacryl has the physical properties ofthermoplastic materials, as indicated by the manufacturer (5). Despite the relatively old age for the use ofthermoplastic denture base materials, reports to evaluate its impact on masticatory muscle activity in patientswearing ISODs are scarce. Versacryl has a compressive or cushioning action. Under functional load, Versacrylabsorbs the applied stress and minimizes the distortion of the supporting tissues. Also, it may reduce the fatigue ofthe masticatory muscles. This, in turn, may positively affect the health of the supporting tissues and result in a bettermasticatory muscle activity. The aim of the current study was to examine this hypothesis. The superficial masseterand anterior fibers of the temporalis muscle were selected to represent the masticatory muscle activity because theyare the largest and strongest masticatory muscles and, thus, have a major role in mandibular movement. They alsoare accessible during EMG records (14). Surface electrodes were preferred over needle electrodes because they areless painful, which, in turn, would affect the EMG records. Surface electrodes allow evaluation of a wider area ofthe muscle (15). Overall, the findings of this study indicated that resilient Versacryl overdentures resulted in asignificant reduction in the activity of the temporalis and masseter muscles. Chewing using the Versacryloverdenture was faster and required fewer chewing strokes. This may result in improvement of the masticatoryfunction and preservation of the masticatory apparatus (4). The mean EMG values of masseter and temporalismuscles were gradually decreased from the time of implant placement until the end of the follow-up period. Thismay be attributed to the adaptability of the neuromuscular system to the overdenture treatment throughout thefollow-up period. Consequently, the effort needed for mastication decreased with time (16). Previous studiesindicated that treatment with ISODs improved the functional state of the masticatory apparatus and aided inestablishment of better neuromuscular coordination by improving retention, support, and stability of the prosthesis,and, consequently, less effort had to be exerted by the muscles to control the prosthesis during function (17, 18). Alimitation of this work is that only male patients were included. Also, this was a short‑term study, and further studiesare required to evaluate the long‑term impact of flexible acrylic resin on masticatory muscle activity and oralcomfort in patients wearing ISODs.

5. ConclusionsThe use of flexible acrylic resin in the construction of ISODs resulted in improved masticatory muscle activity.Therefore, the hypothesis of no significant difference in masticatory muscle activity between mandibular ISODs ofconventional construction and mandibular ISODs made of flexible acrylic resin was rejected.

Acknowledgments:The authors thank the National Research Centre, Cairo, Egypt for supporting this study.

Conflict of Interest:There is no conflict of interest to be declared.

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Authors' contributions:Both authors contributed to this project and article equally. All authors read and approved the final manuscript.

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